1. Field of the Invention
The present invention relates to an image forming apparatus, a fixation device and a heat control method for a fixation device.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, and a facsimile machine, an image is fixed on a recording medium by transcribing a toner image formed on a photoconductor drum onto a transcription paper as a recording medium and, subsequently applying a heating treatment on it by a fixation roller of a fixation device which roller is referred to as a heating roller. In the fixation device, a fixation roller heated by a heat generation member such as a halogen heater and a member heated by means of electromagnetic induction (a roller with heating means is referred to as a fixation roller, below.) and a pressurizing roller arranged to oppose the fixation roller are pressurized by and contacted to each other so as to form a mutual pressurizing and contacting part referred to as a nip part, and the recording medium on which a toner image has been transcribed passes through and heated at the nip part.
A thermistor as a contact-type temperature sensor is configured to contact a fixation roller and fixation failure caused by temperature non-uniformity on the fixation roller is prevented by measuring the temperature of the fixation roller by using the thermistor and controlling the temperature of a heat generation member so that the surface temperature of the fixation roller is constant. However, in the first printing after turning on a power supply, when printing is performed on the condition that a transcription paper and toner have been cooled, the heat of the fixation roller transmits to the transcription paper at the time of fixation and, therefore, the temperature of a nip part is lowered, whereby there is a problem such that non-uniformity of fixation occurs. Particularly, the lowering of the temperature of the nip part is significant in the case of performing printing under a low-temperature environment and non-uniformity of fixation easily occurs.
Commonly, in order to prevent such temperature lowering, it is possible to prevent the temperature lowering through paper passage by increasing the thickness of a fixation roller thereby increasing the heat capacity thereof.
However, if the thickness of the fixation roller is increased, it takes starting time period until it reaches a certain temperature at the time of heat generation and a time period for providing a printable condition is required from turning on a power supply, whereby there is a problem of degrading the use convenience of a user. As a method for reducing the starting time period, there is provided a method for providing a pressurizing roller with an auxiliary heater so as to compensate the temperature lowering caused by paper passage, but, in this case, a certain cost of a heater, a certain capacity of a power supply, and further a driver for driving an auxiliary heater are needed thereby increasing the cost and there is a problem of consuming extra energy.
Then, for example, it is suggested to prevent a temporary lowering of the surface temperature of a fixation roller by heating the fixation roller and a transcription paper using infrared rays generated from a halogen electric bulb in Japanese Laid-Open Patent Application No. 09-054519. Also, it is suggested to prevent a temporary lowering of the surface temperature by heating an intermediate transcriber and toner using a heat pipe in Japanese Laid-Open Patent Application No. 11-065330.
On the other hand, commonly, it is possible to prevent temperature lowering caused by paper passage by increasing the thickness of a fixation roller and increasing the heat capacity thereof in order to prevent the temperature lowering. However, if the thickness of a fixation roller, it takes starting time period until it reaches a certain temperature at the time of heat generation and a time period for providing a printable condition is required from turning on a power supply, whereby the use convenience of a user is degraded. Then, as a method for reducing the starting time period, there is provided a method for providing a pressurizing roller with an auxiliary heater so as to compensate the temperature lowering caused by paper passage. However, in this case, a certain cost of a heater, a certain capacity of a power supply, and further a driver for driving an auxiliary heater are needed thereby increasing the cost and there is a problem of consuming extra energy.
Therefore, it is necessary to reduce starting time period until it reaches a certain temperature at the time of heat generation by reducing the thickness of a fixation roller, but, as described above, particularly in the first printing under a low-temperature environment, when printing is performed on the condition that a transcription paper and toner have been cooled, the temperature of a nip part is lowered, whereby there is a problem such that non-uniformity of fixation occurs. In this case, since the temperature is changed like a transient response, even if the temperature is detected by a thermistor after the temperature is changed, some time period is practically required until the temperature reaches a target temperature by raising heating temperature and it cannot be followed in a response time.
There are problems of increasing the cost as described above in the prevention of temporary temperature lowering at the time of fixation by previously heating a transcription paper, toner or an intermediate transcriber by a heat pipe or other means as the conventional techniques described above, and of increasing a apparatus surface area for configuring them.
Therefore, it is necessary to reduce starting time period required for reaching a certain temperature at the time of heat generation by reducing the thickness of a fixation roller, but, as described above, if printing is performed at the condition that a transcription paper or toner have been cooled, the temperature of a nip part is lowered, whereby there is a problem such that non-uniformity of fixation occurs. In this case, since the temperature is changed like a transient response, the following control is hardly conducted in the transient time by the temperature control using a thermistor.
As notice of the problem of such a conventional technique is taken, it is desired to suppress degradation of image quality caused by temporal lowering of surface temperature of a fixation roller or degradation of image quality under a lower temperature environment.
Meanwhile, as such a technique, for example, a conventional technique disclosed in Japanese Laid-Open Patent Application No. 2002-237377 is known. The conventional technique disclosed in Japanese Laid-Open Patent Application No. 2002-237377 aims at providing a fixation device with a high fixation performance which satisfies the reduction of reactive current provided on commercial alternating current, the prevention of audible frequency noise, constant heating output with an inexpensive structure and an image forming apparatus with a high fixation performance and little image quality degradation. Then, it is an electrically inductive heating method which chops and applies direct current obtained by rectification of alternating current to a resonant circuit including an electric coil arranged in vicinity to an object to be heated and a capacitor for resonance connected thereto by the repetition of switching on and off of a switching element, characterized by detecting the variation of a voltage between the resonant circuit and the switching element so as to switch on the switching element in synchronization with it and detecting current through the switching element so that it switches off the switching element after an on-time based on the time required for reaching an instantaneous value of an envelop level proportional to a voltage wave pattern obtained by the rectification of the alternating current.
Meanwhile, an inverter is used in the conventional technique which includes a fixation device disclosed in Japanese Laid-Open Patent Application No. 2002-237377. In such an inverter, a constant OFF time period (P) is necessarily retained and then the duration of an ON time period is adjusted to conduct the electrically inductive heating of a load, herein a heated part of a fixation roller, as shown in FIGS. 13 (a) and (b).
However, when driving pulses as shown in FIG. 13 (b) is turned off, a resonance voltage is generated between both terminals of a switching element. Commonly, a control to retain an OFF time period (P) which is longer than the duration time of the resonance voltage is conducted. The relationship between the resonance voltage and the switching off in this case is shown in FIGS. 14 (a) and (b). Thus, when the control to retain the OFF time period (P) is conducted and a control part encounters a difficulty, and further, when the switch is turned on at the time of generation of a resonance voltage as shown in FIG. 14 (b), the resonance is consequently enhanced and a lot of stress is generated in the switching element, which may lead to the breaking thereof if the worst happens.
As notice of the problem of such a conventional technique is taken, it is desired to prevent a switching element from breaking.